Not applicable.
Not applicable.
The invention relates to a printing apparatus for objects to be printed, of the generic type having a circular indexing table which rotates about a vertical axis and has a rotary drive, having at least one pallet carrier arranged on the circular indexing table and in each case having a pallet that can be arranged on the pallet carrier, at least one printing unit arranged above the pallets circulating around the vertical axis, the printing unit and the pallet being arranged such that they can move relative to each other in the vertical direction, and the pallet carrier arranged on the circular indexing table assuming a plurality of electrically indexable, stepwise predefinable rotary positions with respect to the printing unit.
In order to print objects, in particular T-shirts or textiles, these are fixed to pallets which are arranged on a circular indexing table in the form of a carousel which has a plurality of pallet carriers and rotates about a vertical axis. Above the pallets, at least one printing unit is arranged coaxially with the vertical axis. By means of a lifting apparatus, the pallets and the printing unit can be moved vertically relative to each other, so that an object can be printed by moving the pallet and printing unit together.
In order to apply multicolour prints to an object, for example in the screen printing process, it is necessary, in order to obtain a specific colour, for the respective primary colours to be applied to the object one after another and as precisely above one another as possible in operations which proceed one after another. The precise application of a plurality of printing inks one after another is referred to as register accuracy.
In the known printing apparatuses of the generic type, the individual rotary positions, in which in each case a printing unit for an individual colour, a drying device, a feeder station or a discharge station for the objects to be printed are arranged, are moved to little by little at constant rotational speed. For this purpose, use is made of external sensors which monitor and control the horizontal positioning of the printing units relative to the circular indexing table. The production of such a printing apparatus is very expensive and the maintenance is very costly, since the sensors and transmitters used are casily contarninated. Furthermore, in the known printing apparatuses of the generic type, it is disadvantageous that the individual rotary positions have to be moved to little by little at constant speed, the residence time in the respective positions being predefined by the position to be moved through most slowly.
The invention is therefore based on the object of providing a printing apparatus in which, firstly, a relatively high operating speed is possible and, secondly, the expenditure on maintenance and the susceptibility to faults are reduced considerably.
The printing apparatus according to the invention of the generic type of the main claim, as compared with the prior art, has the advantage that the rotary drive has an electric motor, the electric motor being an asynchronous motor or a servo motor which is connected to a frequency converter or to a servo converter, so that the rotational speed and the direction of rotation of the electric motor can be controlled by a control apparatus, by the frequency and/or the voltage of the frequency converter or of the servo converter being controlled, a rotary encoder, for example an optical, a mechanical or an inductive sensor, being arranged directly on the electric motor, preferably on its drive shaft, so that the printing apparatus and the circular indexing table can be positioned coarsely relative to each other in terms of their rotary position and, when moved vertically together, the register accuracy required for a multicolour print being achieved by means of a mechanical fine positioning apparatus arranged in at least one rotary position on the circular indexing table and on the printing unit that can be moved vertically relative thereto. The control apparatus of the printing apparatus according to the invention can in this case be programmed in such a way that the individual rotary positions can be skipped. Since all the primary colours kept ready in mutually separated printing units are not always needed for a colour to be mixed, in this way rotary stations under the printing units and having a colour that is not needed can be skipped as required, which means that the operating speed of the printing apparatus according to the invention is increased as compared with the prior art. The control of the rotary position of the circular indexing table permits the programming of the control apparatus to the effect that shorter rotary paths between rotary positions with the respective processing steps that follow after one another during printing can be shortened and in this way the operating speed of the printing apparatus can be increased. By connecting the control apparatus to the lifting device for the vertical relative movement between printing unit and circular indexing table, when individual rotary positions are skipped, the vertical relative movements are not carried out, which achieves additional acceleration of the operating speed of the printing apparatus according to the invention. As a result of using an internal rotary encoder arranged directly on the electric motor, the external sensors which were used hitherto and which, for example, need toothed belts and tensioning rollers, are dispensed with, which makes a simpler, cheaper and more reliable construction possible. As compared with the prior art, the rotary encoder arranged directly on the electric motor is better protected, has no play and needs no readjustment. The control of the frequency and/or of the voltage of the frequency converter or the servo converter allows the rotational speed to be changed during the operation of the printing apparatus according to the invention, as a result of which, for example, rapid rotation during the transport between two positions and slow rotation, for example under an infrared drying station, are possible.
One refinement of the invention provides for the motor to have a reduction gearbox, whose smaller gear meshes with a larger gear, and for the larger gear to be connected concentrically rigidly to the vertical axis of rotation of the apparatus.
Another refinement of the invention provides for the gears to have helical toothing.
An additional refinement of the invention provides for the frequency to be controllable, preferably in the range from 0 to 80 Hz.
Another, additional refinement of the invention provides for the frequency to be controllable in decade stages.
An additional refinement of the invention provides for the voltage to be controllable, preferably in a range between 1 and 10 V.
An additional refinement of the invention provides for the printing apparatus to be capable of being driven with multiple indexing, so that a plurality of rotary positions can be skipped one after another or in a specific order.
An additional refinement of the invention provides for the printing apparatus to be capable of being driven with double indexing, so that two rotary positions are skipped, it being possible for identical production sequences to proceed in adjacent rotary positions.
An additional refinement of the invention provides that, in the case of double printing systems, the stencils used in screen printing are twice as large as standard, since larger stencils are possible as a result of skipping rotary positions.
An additional refinement of the invention provides for the stencil size to be 500 mmxc3x971000 mm in double printing systems.
Another refinement of the invention provides for the speed to be slow in intermediate printing stations.
An additional refinement of the invention provides for the printing apparatus to have twelve divisions or eight divisions or six divisions so that, on such a printing apparatus, twelve, eight or six processing stations, including the positions for the fitting and removal of the pallets, can be arranged, the mechanical mimic for the six, eight or twelve divisions being designed identically, so that in the case of different objects to be printed, it is merely necessary for a different program to be called in the control apparatus, and no time-consuming rebuilding of the mechanism is required.